The present invention relates generally to natural gas engines, and more specifically relates to an air-fuel control system for natural gas engines.
Natural gas engines are used today as an alternative to liquid fuel engines. The overall performance of the engine in terms of combustion efficiency, speed control, exhaust emission of pollutants and others, greatly depends on controlling the mixing of the air and fuel into an appropriate ratio for combustion and on regulating the flow of this mixture into the combustion part of the engine. Air-fuel control refers herein to both of these control processes.
Many natural gas engines use one or more air-fuel mixing devices such as a carburetor, to prepare a suitable air-fuel mixture for being combusted in the engine. FIG. 1 shows a schematic view of a general natural gas engine with a carburetor using gaseous fuel. Examples of gaseous fuels are methane, ethane, propane, butane, carbon monoxide and hydrogen and mixtures of these gases. The combustion occurs in the engine cylinders. The air-fuel mixture is supplied to the cylinders from the intake manifold. For each cylinder head, an intake valve in the cylinder head opens to admit the air-fuel mixture from the intake manifold into each cylinder. After combustion, an exhaust valve in each cylinder head opens to allow the gaseous combustion products to flow to the exhaust manifold.
The air-fuel mixture flows into the intake manifold from the carburetor of the engine through a throttle or butterfly valve. A governor control unit controls the throttle valve either mechanically or electronically, such that if the engine speed is below the desired speed, the throttle is opened to admit more of the air-fuel mixture from the carburetor and vice-versa.
Air and fuel are supplied to the carburetor for being mixed in a controlled ratio. The flow of fuel into the carburetor is controlled by adjusting fuel valves upstream of the carburetor. The carburetor is designed to control the fuel flow according to the flow of air, such that as the air flow increases more fuel is added.
Air to the carburetor comes either directly from the atmosphere, as naturally aspirated, or from an air pressure increasing device, such as a blower, supercharger, turbocharger or fan, which increases the pressure and density of the air. Usually, a turbocharger is powered by the hot exhaust gases, while a supercharger is driven mechanically by the engine or a separate motor, such as an electric motor. Not shown, but often present, is an air cooler between the air pressure increasing device and the carburetor or air-fuel mixing device. The fuel gas comes from a fuel source and flows through a pressure regulator that ensures the pressure of the gas provided to the carburetor is controlled. In some cases the regulator is referenced to the air pressure of the air provided to the carburetor.
The combustion gases may either be vented to the atmosphere or be ducted to a turbocharger, which derives energy from the hot exhaust gases to compress the air used by the engine.
For multi-cylinder engines and V engines there may be more than one carburetor and one turbocharger or supercharger. The control method is similar to that described.
Many special modifications have been made to the carburetor design to achieve the desired air to fuel control over the starting and running conditions for a wide load range. In practice, it has proven very difficult to achieve the desired result over all conditions. Normally, a best compromise has been used according to the particular demands of the operating requirements for the engine. In addition, engine starting is sometimes difficult due to the inability to achieve the desired air-fuel control for starting and idle conditions.
It is an object of the invention to provide a novel air-fuel control system for natural gas engines.
According to one aspect of the invention, there is provided a method for controlling the air-fuel ratio in a natural gas engine having an air-fuel mixing device, the method comprising: determining a speed set-point and an actual engine speed; determining a fuel command based on said speed set-point and said engine speed; and controlling the fuel flow to the air-fuel mixing device in response to said fuel command.
According to another aspect of the invention, there is provided an air-fuel control system for natural gas engines, the air-fuel control system comprising: means to determine a speed set-point and an actual engine speed; a governor control unit for generating a fuel command based on said speed set-point and said actual engine speed; a fuel flow regulator coupled to said governor control unit for regulating the flow of fuel to an air-fuel mixing device in response to said fuel command.
The invention described can be applied to any engine currently equipped with one or more air-fuel mixing devices such as a carburetor.
Among the advantages presented by the air-fuel control system of the preferred embodiment of the invention are: good speed control, stable operation, ability to follow load changes and low emissions of CO and NOX. Other advantages, objects, and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of preferred embodiments in conjunction with the accompanying drawings and claims.